JP7240615B2 - Negative electrode for lithium ion secondary battery and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a negative electrode for a lithium ion secondary battery and a manufacturing method thereof.

In recent years, lithium-ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, etc., and power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). ing.

A negative electrode used in a lithium ion secondary battery typically has a structure in which a negative electrode active material layer is provided on a negative electrode current collector. A negative electrode active material layer is generally produced using a negative electrode paste containing a negative electrode active material and a binder (see, for example, Patent Document 1). In Patent Document 1, the negative electrode paste has a zeta potential of −16.78 to −4.83 mV and a conductivity of 0.48 to 0.65 S·m ⁻¹ to coat the surface of the negative electrode active material. However, a technique for optimizing the amount of water-soluble polymer in the thickener is described. Patent Literature 1 describes that this technology can suppress deposition of metallic lithium on the negative electrode due to excessive water-soluble polymer coating the surface of the negative electrode active material.

JP 2009-224099 A

However, according to the studies of the present inventors, there are factors other than those described in the prior art for the deposition of metallic lithium. Found it.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for manufacturing a negative electrode for a lithium ion secondary battery having excellent resistance to lithium deposition.

A method for manufacturing a negative electrode for a lithium ion secondary battery disclosed herein includes the steps of preparing a negative electrode paste containing a negative electrode active material and a binder, and fabricating a negative electrode using the negative electrode paste. A ratio (ζ _B /ζ _A ) of the zeta potential (ζ _B ) of the binder to the zeta potential (ζ _A ) of the negative electrode active material is 3.5 or more and 9.0 or less.
According to such a configuration, a method for manufacturing a negative electrode of a lithium ion secondary battery having excellent resistance to lithium deposition is provided.

In a preferred aspect of the method for manufacturing a negative electrode for a lithium ion secondary battery disclosed herein, the negative electrode active material has a zeta potential (ζ _A ) of −3.1 mV or more and −5.5 mV or less.
According to such a configuration, the lithium ion deposition resistance becomes higher.
In a preferred embodiment of the method for producing a negative electrode of a lithium ion secondary battery disclosed herein, the negative electrode paste further contains ceramic particles exhibiting a zeta potential of −25 mV or less in a pH range of 8-9.
According to such a configuration, the lithium ion deposition resistance becomes higher.

From another aspect, the negative electrode of the lithium ion secondary battery disclosed herein includes a negative electrode active material and a binder. The average diameter of the binder is 0.2 μm or more and 0.5 μm or less, and the peak half width of the particle size distribution curve of the binder is 0.40 μm or more and 0.65 μm or less.
According to such a configuration, a negative electrode for a lithium ion secondary battery having excellent resistance to lithium deposition is provided.

1 is a cross-sectional view schematically showing the internal structure of a lithium ion secondary battery using a negative electrode obtained by a manufacturing method according to an embodiment of the invention; FIG. 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery using a negative electrode obtained by a manufacturing method according to one embodiment of the present invention; FIG. 4 is a backscattered electron image showing the dispersed state of the negative electrode binder obtained in Example 1. FIG. 4 is a backscattered electron image showing the dispersed state of the negative electrode binder obtained in Comparative Example 1. FIG. 4 is a graph showing charge/discharge patterns when measuring rated capacity in evaluation of lithium deposition resistance.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for the implementation of the present invention can be grasped as design matters by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Further, in the following drawings, members and portions having the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors.
In this specification, the term “lithium ion secondary battery” refers to a secondary battery that utilizes lithium ions as a charge carrier and is charged/discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

A method for manufacturing a negative electrode for a lithium ion secondary battery according to the present embodiment includes a step of preparing a negative electrode paste containing a negative electrode active material and a binder (hereinafter also referred to as a “negative electrode paste preparation step”), and using the negative electrode paste. and a step of manufacturing a negative electrode (hereinafter also referred to as a “negative electrode manufacturing step”). Here, the ratio (ζ _{B /ζ A ) of the zeta potential (ζ B ) of} the binder to the zeta potential (ζ _A ) of the negative electrode active material is 3.5 or more and 9.0 or less.
As used herein, the term "paste" refers to a mixture in which part or all of the solid content is dispersed in a solvent, and includes so-called "slurry", "ink", and the like.

First, the step of preparing the negative electrode paste will be described. The negative electrode paste prepared in this step contains at least a negative electrode active material and a binder.
The type of negative electrode active material is not particularly limited as long as the ratio (ζ _B /ζ _A ) is 3.5 or more and 9.0 or less. Carbon materials such as graphite, hard carbon, and soft carbon can be suitably used as the negative electrode active material, and among these, graphite is preferred. Graphite may be natural graphite or artificial graphite. In addition, amorphous carbon-coated graphite, in which the surface of graphite is coated with an amorphous carbon film, may be used.

Although the zeta potential of the negative electrode active material is not particularly limited, it is preferably −3.1 mV or more and −5.5 mV or less. When the zeta potential of the negative electrode active material is within this range, aggregation of the binder can be further suppressed, and the lithium deposition resistance of the negative electrode can be further enhanced.
Here, the zeta potential of graphite is usually -2 mV or higher. Therefore, in the present embodiment, graphite particles that have undergone processing to reduce the value of zeta potential (that is, to increase the absolute value) are preferably used.
As a method for reducing the value of zeta potential, there is a method of treating graphite with H ₂ O plasma. By treating graphite with H ₂ O plasma, the amount of surface hydroxyl groups increases, so the value of zeta potential can be reduced. Furthermore, the zeta potential can be easily adjusted depending on the plasma treatment conditions. Although the amount of surface hydroxyl groups of this graphite is not particularly limited, it is preferably 0.21 mmol/g or more and 0.30 mmol/g.
The zeta potential of the negative electrode active material can be obtained, for example, by measuring a sample prepared by dispersing the negative electrode active material at a concentration of 0.05 g/L in ion-exchanged water using an electrophoretic light scattering method. can.

From this point of view, graphite with a zeta potential of −3.1 mV or more and −5.5 mV or less is proposed here. In particular, graphite having a zeta potential of −3.1 mV or more and −5.5 mV or less and a surface hydroxyl group content of 0.21 mmol/g or more and 0.30 mmol/g is proposed here. The amount of surface hydroxyl groups can be determined, for example, by neutralization titration.
Also proposed herein is a method for producing a negative electrode active material comprising H ₂ O plasma treatment of graphite. The H ₂ O plasma treatment can be performed with a known plasma treatment apparatus using H ₂ O as a gas species.

The average particle size of the negative electrode active material is not particularly limited, and is, for example, 50 μm or less, typically 1 μm or more and 20 μm or less, preferably 5 μm or more and 15 μm or less.
In the present specification, the "average particle size" refers to the particle size (D50) at which the cumulative frequency is 50% in volume percentage in the particle size distribution measured by the laser diffraction scattering method, unless otherwise specified. say.

The type of binder is not particularly limited as long as the ratio (ζ _B /ζ _A ) is 3.5 or more and 9.0 or less. A rubber-based binder can be preferably used as the binder. Examples thereof include styrene-butadiene rubber (SBR) and modified products thereof, acrylonitrile-butadiene rubber and modified products thereof, acrylic rubber and modified products thereof, and fluororubbers. Among them, SBR is preferable.

Here, the zeta potential of the binder is not particularly limited. Rubber-based binders with different zeta potentials are known. In addition, rubber-based binders are often copolymerized with a small amount of an acid monomer or an acrylic acid ester in order to improve water dispersibility. can be done.
The zeta potential of the binder can be obtained, for example, by measuring a sample prepared by dispersing the binder at a concentration of 0.05 g/L in ion-exchanged water by an electrophoretic light scattering method.

In the present embodiment, the ratio (ζ _{B /ζ A ) of the zeta potential (ζ B ) of} the binder to the zeta potential (ζ _A ) of the negative electrode active material is 3.5 or more and 9.0 or less.
According to studies by the present inventors, one of the factors for the deposition of metallic lithium in the negative electrode of a lithium ion secondary battery is that the binder aggregates in the negative electrode active material layer, and when the aggregated binder adheres to the negative electrode active material, the resistor I found out that there is something to be. Here, the zeta potential is a parameter related to the surface potential of the particles. Therefore, if the ratio (ζ _{B /ζ A ) of the zeta potential (ζ B ) of} the binder to the zeta potential (ζ _A ) of the negative electrode active material is within the above range, the charged state of the negative electrode active material and the binder in the negative electrode paste becomes appropriate, the dispersion state of the negative electrode active material and the binder in the negative electrode paste is improved, and aggregation of the binder can be suppressed. As a result, the lithium deposition resistance of the negative electrode can be improved.

A negative electrode paste usually contains a solvent. As the solvent, an aqueous solvent is preferably used. An aqueous solvent refers to water or a mixed solvent mainly composed of water. Solvents other than water that constitute the mixed solvent include organic solvents that can be uniformly mixed with water (eg, alcohols having 4 or less carbon atoms, ketones having 4 or less carbon atoms, etc.). The aqueous solvent preferably contains 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more of water. Water is most preferred as the solvent.

The negative electrode paste may contain components other than those described above. Examples include thickeners, ceramic particles, pH modifiers, and the like. Examples of thickeners include cellulose-based polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropylmethylcellulose (HPMC), and polyvinyl alcohol (PVA). Among them, CMC is preferred.

Ceramic particles that do not participate in charge-discharge reactions are preferred, and examples thereof include alumina, boehmite, and aluminum hydroxide. Ceramic particles are generally much harder than the negative electrode active material, which is a carbon material. Deformation (expansion and compression) of the active material layer is suppressed, and cycle characteristics can be improved.
Among ceramic particles, ceramic particles exhibiting a zeta potential of -25 mV or less in the pH range of 8 to 9 are preferred. When the negative electrode paste contains such ceramic particles, lithium deposition resistance can be further improved. This improves the filling state of the ceramic particles between the negative electrode active material and between the negative electrode and the binder, and makes it possible to follow the deformation (expansion and compression) of the negative electrode active material layer during charging and discharging of the lithium ion secondary battery. This is thought to be for improvement.
Note that the zeta potential of the ceramic particles was determined, for example, by dispersing the ceramic particles in ion-exchanged water at a concentration of 0.05 g/L and adjusting the pH to a range of 8 to 9 with an aqueous lithium hydroxide solution. It can be obtained by measuring by the electrophoretic light scattering method.
The average particle size of the ceramic particles is not particularly limited, but is, for example, 0.05 μm or more and 3 μm or less, preferably ⅕ or less of the average particle size of the negative electrode active material.

The content of the negative electrode active material in the total solid content of the negative electrode paste is preferably 50% by mass or more, more preferably 90% by mass or more and 99.5% by mass or less, and still more preferably 95% by mass or more and 99% by mass or less. .
The content of the binder in the total solid content of the negative electrode paste is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.3% by mass or more and 5% by mass or less, and still more preferably 0.5% by mass. % by mass or less.
The content of the thickener in the total solid content of the negative electrode paste is preferably 0.3% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 2% by mass or less.
The content of the ceramic particles in the total solid content of the negative electrode paste is preferably 0.5% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 15% by mass or less.

The solid content concentration of the negative electrode paste is, for example, 40 mass % or more, preferably 45 mass % or more and 80 mass % or less, more preferably 50 mass % or more and 60 mass % or less. When the solid content concentration is within the above range, the drying efficiency of the negative electrode paste can be improved. In addition, handling of the negative electrode paste becomes easy, uniform coating becomes easy, and formation of a negative electrode active material layer having a uniform thickness becomes easy.

A negative electrode paste can be prepared by mixing a negative electrode active material, a binder, a solvent, and optional components according to a known method. As an example, first, the negative electrode active material and the thickening agent are dry-mixed, and then a part of the solvent is added and moistened. After kneading if necessary, the remainder of the solvent is added for dilution. A binder can be put into the mixture and stirred to obtain a negative electrode paste. As another example, the negative electrode active material and the thickener are dry-mixed, and the mixture is kneaded after the total amount of the solvent is added. A binder can be put into the mixture and stirred to obtain a negative electrode paste. When using ceramic particles that exhibit a zeta potential of -25 mV or less in the pH range of 8 to 9, the pH of the negative electrode paste is adjusted to, for example, 8 to 9 with a pH adjuster (eg, lithium hydroxide, etc.). is preferred.

Next, the negative electrode manufacturing process will be described.
There is no particular limitation on the method for producing the negative electrode using the negative electrode paste, but the negative electrode production process is typically
A step of applying the negative electrode paste to the negative electrode current collector (hereinafter also referred to as a “paste coating step”); and a step of drying the applied negative electrode paste to form a negative electrode active material layer (hereinafter referred to as a “drying step ”)
can be done by implementing
A step of pressing the negative electrode active material layer (hereinafter also referred to as a “pressing step”) may be performed after the drying step.

The paste coating process will be described.
As the negative electrode current collector, a conductive member made of a highly conductive metal (for example, copper, nickel, titanium, stainless steel, etc.) is typically used. The shape of the negative electrode current collector is not particularly limited, and may be rod-like, plate-like, sheet-like, foil-like, mesh-like, or the like.
The negative electrode current collector is preferably copper foil.
When the negative electrode current collector is a copper foil, its thickness is not particularly limited, but is, for example, 6 μm or more and 30 μm or less.

Application of the negative electrode paste to the negative electrode current collector can be performed according to a known method. For example, it can be carried out by applying the negative electrode paste onto the negative electrode current collector using a coating device such as a gravure coater, a comma coater, a slit coater, a die coater, or the like. The negative electrode active material layer may be formed only on one side of the negative electrode current collector, or may be formed on both sides, preferably on both sides. Therefore, the negative electrode paste is applied on one side or both sides of the negative electrode current collector, preferably on both sides.

Next, the drying process will be explained. The drying step can be performed according to a known method. For example, it can be carried out by removing the solvent from the negative electrode current collector coated with the negative electrode paste using a drying device such as a drying oven. The drying temperature and drying time may be appropriately determined according to the type of solvent used, and are not particularly limited. The drying temperature is, for example, over 70° C. and 200° C. or less (typically 110° C. or more and 150° C. or less). The drying time is, for example, 10 seconds or more and 600 seconds or less (typically 30 seconds or more and 300 seconds or less).
By performing the drying step, a negative electrode active material layer can be formed on the negative electrode current collector.

Next, the press process will be explained. The pressing step can be performed according to a known method. For example, it can be carried out by subjecting the negative electrode active material layer formed as described above to press treatment such as roll pressing. By carrying out the pressing step, the thickness, basis weight, density, etc. of the negative electrode active material layer can be adjusted.

A negative electrode can be manufactured in the manner described above.
The negative electrode produced in this manner is excellent in lithium deposition resistance. This is because aggregation of the binder is suppressed as described above, and in the negative electrode thus produced, the binder is uniformly dispersed in the form of fine particles.
Specifically, since aggregation of the binder is suppressed, the average diameter of the binder is 0.2 μm or more and 0.5 μm or less, and the peak half width is 0.40 μm or more and 0.5 μm or less in the particle size distribution curve of the binder. A dispersion of 6 μm or less can be achieved.
Therefore, from another aspect, a negative electrode of a lithium ion secondary battery having excellent lithium deposition resistance includes a negative electrode active material and a binder, the average diameter of the binder is 0.2 μm or more and 0.5 μm or less, and the particle size of the binder is A negative electrode for a lithium-ion secondary battery is proposed herein, wherein the peak half width in the distribution curve is 0.40 μm or more and 0.65 μm or less.

Note that the average diameter of the binder and the particle size distribution curve can be obtained using, for example, a cross-sectional SEM image of the negative electrode active material layer. Specifically, for example, it can be obtained as follows.
A sample of the negative electrode active material layer is prepared, and the binder is dyed with Os using OsO ₄ if necessary. A cross section of the negative electrode active material layer is observed with a scanning electron microscope (SEM) to obtain a backscattered electron image. A backscattered electron image is binarized with the binder and the rest. The diameter of the binder clarified by the binarization process is measured. The diameter of the binder is calculated as the diameter of a circle having the same area as the image of the binder (so-called equivalent circle diameter), and the average value is obtained as the average diameter. Commercially available software may be used for this measurement. Also, based on the obtained binder diameter data, a particle size distribution curve is created with the vertical axis representing the frequency and the horizontal axis representing the binder diameter. From this curve, the peak half width is obtained by obtaining the peak width at half the height of the peak.

By using the negative electrode produced by the manufacturing method according to the present embodiment in a lithium ion secondary battery, it is possible to provide a lithium ion secondary battery having excellent resistance to lithium deposition.
Therefore, an example of the configuration of a lithium ion secondary battery manufactured using the negative electrode obtained by the manufacturing method according to the present embodiment will be described below with reference to FIGS. 1 and 2. FIG. In addition, the lithium ion secondary battery configured using the negative electrode obtained by the manufacturing method according to the present embodiment is not limited to the following examples.

The lithium ion secondary battery 100 shown in FIG. 1 is a closed type constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte 80 in a flat rectangular battery case (that is, an outer container) 30. Battery. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. ing. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80 . The positive terminal 42 is electrically connected to the positive collector plate 42a. The negative terminal 44 is electrically connected to the negative collector plate 44a. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used.

As shown in FIGS. 1 and 2, the wound electrode body 20 is formed by stacking a positive electrode sheet 50 and a negative electrode sheet 60 with two long separator sheets 70 interposed therebetween and winding them in the longitudinal direction. morphology. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long positive electrode current collector 52 . The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long negative electrode current collector 62 . The positive electrode active material layer non-formed portion 52a (that is, the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the negative electrode active material layer non-formed portion 62a (that is, the negative electrode active material layer 64 is formed). The portion where the negative electrode current collector 62 is exposed without being wound) is formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axial direction (that is, the sheet width direction orthogonal to the longitudinal direction). there is A positive collector plate 42a and a negative collector plate 44a are joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a, respectively.

Examples of the positive electrode current collector 52 forming the positive electrode sheet 50 include aluminum foil. Examples of the positive electrode active material contained in the positive electrode active material layer 54 include lithium transition metal oxides (eg, LiNi1 _/ 3Co1/ _{3Mn1 / 3O2} , _LiNiO2 , _LiCoO2 , _LiFeO2 , _{LiMn2O 4} , _{LiNi0.5Mn1.5O4 , etc.} ) and lithium transition metal phosphate compounds (eg, _{LiFePO4 ,} etc.). The positive electrode active material layer 54 may contain components other than the active material, such as a conductive material and a binder. Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used.

For the negative electrode sheet 60, the negative electrode obtained by the manufacturing method according to the present embodiment is used.

Examples of the separator 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70 .

The non-aqueous electrolyte 80 can be the same as that used in conventional lithium ion secondary batteries, and typically an organic solvent (non-aqueous solvent) containing a supporting electrolyte can be used. As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in electrolytes of general lithium ion secondary batteries, can be used without particular limitation. can be done. Among them, carbonates are preferable, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC ), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC) and the like. Such non-aqueous solvents can be used singly or in combination of two or more. Lithium salts such as LiPF ₆ , LiBF ₄ and LiClO ₄ (preferably LiPF ₆ ) can be suitably used as the supporting salt. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

The non-aqueous electrolyte includes, for example, gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); film-forming agents such as oxalato complex compounds containing boron atoms and/or phosphorus atoms and vinylene carbonate (VC). a dispersant; and various additives such as a thickener.

The lithium ion secondary battery 100 configured as described above can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). The lithium ion secondary battery 100 can also be used typically in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

As an example, a prismatic lithium ion secondary battery having a flat wound electrode assembly has been described. However, the configuration of the lithium ion secondary battery including the negative electrode obtained by the manufacturing method according to this embodiment is not limited to this. The lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode body, such as a coin-shaped lithium ion secondary battery, a cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, and the like. It can also be configured as

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

[Examples 1 to 4 and Comparative Examples 1 to 5]
<Preparation of negative electrode active material and binder>
Graphite with a zeta potential of −1.2 mV was prepared. The average particle size (D50) of this graphite was 8 µm.
200 g of this graphite was put into a chamber type plasma generator. Graphite with a zeta potential of −3.1 mV was obtained by plasma treatment for 90 minutes with H ₂ O as the gas species and the internal pressure of the chamber at 20 Pa.
Furthermore, by lengthening the H ₂ O plasma treatment time, graphite with a zeta potential of −5.5 mV and graphite with a zeta potential of −8.8 mV were obtained.
Five types of commercially available SBR were prepared. Their zeta potentials were -5 mV, -11 mV, -17 mV, -28 mV, -33 mV.
The zeta potentials of graphite and binder were measured as follows. A sample was prepared by dispersing graphite or a binder in deionized water at a concentration of 0.05 g/L. The zeta potential of this sample was measured using a zeta potential measurement system "ELSZ-2000Z" (manufactured by Otsuka Electronics).
In addition, the amount of surface hydroxyl groups of each graphite was determined by neutralization titration. Table 1 shows the results.

<Production of negative electrode>
Graphite having the zeta potential shown in Table 1 and carboxymethyl cellulose (CMC) as a thickening agent were dry mixed and then water was added. After kneading this, styrene-butadiene rubber (SBR) having a zeta potential shown in Table 1 was added and stirred to prepare a negative electrode paste. The ratio of each component in the solid content was graphite:SBR:CMC=98:1:1 (mass ratio).
The obtained negative electrode paste was coated on both sides of a long copper foil having a thickness of 10 μm in a strip shape, dried, and then roll-pressed to prepare a negative electrode sheet.

<Binder dispersion state evaluation>
The obtained negative electrode sheet was cut, and the binder of the cross section was dyed with OsO ₄ using OsO 4 . A cross section of the negative electrode active material layer was observed with a scanning electron microscope (SEM) to obtain a backscattered electron image. A backscattered electron image was binarized with the binder and the rest. The diameter of the binder clarified by the binarization process was measured. The diameter of the binder was calculated as the diameter of a circle having the same area as the image of the binder (so-called equivalent circle diameter), and the average value was determined as the average diameter. Commercially available software (Jimage-fiji) was used for this measurement.
Further, based on the obtained data of the diameter of the binder, a particle size distribution curve was created with the frequency on the vertical axis and the diameter of the binder on the horizontal axis. From this curve, the peak half width was obtained by obtaining the peak width at half the height of the peak. Table 1 shows the results.
For reference, backscattered electron images of the negative electrodes of Example 1 and Comparative Example 1 are shown in FIGS.

<Production of lithium-ion secondary battery for evaluation>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, LNCM: AB : PVdF = 92:5:3 and mixed with N-methylpyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of a long aluminum foil having a thickness of 15 μm in a width of 100 mm, dried, and then roll-pressed to a predetermined thickness to prepare a positive electrode sheet.
A separator sheet was prepared by forming a 4 μm-thick ceramic layer (heat-resistant layer) on the surface of a 24 μm-thick porous polyolefin sheet having a three-layer structure of PP/PE/PP.
The positive electrode sheet and the negative electrode sheet prepared above were opposed to each other with a separator sheet interposed therebetween to prepare an electrode assembly. The heat-resistant layer of the separator sheet was opposed to the negative electrode.
A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of EC:DMC:EMC=3:3:4 was mixed with LiPF ₆ at a concentration of 1.0 mol/L. A non-aqueous electrolytic solution was prepared by dissolving with
An aluminum case including a lid body having a liquid inlet and a case body was prepared.
An electrode terminal and a collector plate were attached to the lid. Next, the produced electrode body and current collector plate were joined by welding. The electrode body thus joined to the lid was inserted into the case body, and the lid and the case body were welded together.
A predetermined amount of the non-aqueous electrolyte prepared above was injected through the injection port, and the injection port was sealed by tightening a sealing screw.
A lithium ion secondary battery for evaluation was obtained by allowing the electrode body to stand still for a predetermined time so that the non-aqueous electrolyte was impregnated into the electrode body.
Next, each lithium ion secondary battery for evaluation was subjected to initial charging. Specifically, the lithium ion secondary battery prepared above was subjected to constant current charging up to 4.1 V at a current value of 1/3 C, and then constant voltage charging until the current value became 1/50 C. put into charging state.
Then, aging treatment was performed for 20 hours in a constant temperature bath at 50°C.

<Evaluation of resistance to lithium deposition>
The lithium ion secondary batteries for evaluation of each example and each comparative example were adjusted to an SOC of 56% and placed in a temperature environment of -10°C.
1000 charge/discharge cycles were repeated, each cycle consisting of constant current charging at 20C for 30 seconds, resting for 10 minutes, constant current discharging at 20C for 30 seconds, and resting for 10 minutes.
Before and after the 1000 cycles of charging and discharging, the charging and discharging pattern shown in FIG. 5 (energizing current: 0.5 C) was performed, and the discharge capacity during the period indicated by arrow D was measured as the rated capacity.
The capacity retention rate (%) was obtained from (rated capacity after cycle charge/discharge/rated capacity before cycle charge/discharge)×100. Table 1 shows the results.

As the results in Table 1 show, the capacity retention rate was high in Examples 1 to 4 within the scope of the manufacturing method according to the present embodiment. Since the decrease in capacity is caused by the deposition of metallic lithium, the higher the capacity retention rate, the higher the resistance to lithium deposition. Therefore, according to the production method disclosed herein, it is possible to produce a negative electrode for a lithium-ion secondary battery having excellent resistance to lithium deposition.

[Examples 5 to 8]
After dry mixing graphite with a zeta potential of −3.1 mV, carboxymethyl cellulose (CMC) as a thickener, and ceramic particles (CP) shown in Table 2, water and water so that the pH is 9 and an aqueous lithium oxide solution were added. After kneading this, SBR with a zeta potential of −17 mV was added and stirred to prepare a negative electrode paste. The ratio of each component in the solid content was C:SBR:CMC:CP=88:1:1:10 (mass ratio).
The zeta potential of ceramic particles was measured as follows. A dispersion was prepared by dispersing ceramic particles in deionized water at a concentration of 0.05 g/L, and a lithium hydroxide aqueous solution was used to adjust the pH to a range of 8 to 9 to prepare a sample. The zeta potential of this sample was measured using a zeta potential measurement system "ELSZ-2000Z" (manufactured by Otsuka Electronics).
The obtained negative electrode paste was coated on both sides of a long copper foil having a thickness of 10 μm in a strip shape, dried, and then roll-pressed to prepare a negative electrode sheet.

Using the produced negative electrode sheet, a lithium ion secondary battery for evaluation was produced in the same manner as described above, and evaluation of lithium deposition resistance (measurement of capacity retention rate) was performed. Table 2 shows the results.

The results in Table 2 show that the lithium deposition resistance can be further improved by using ceramic particles exhibiting a zeta potential of -25 mV or less in the pH range of 8-9.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 Wound electrode assembly 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formation portion 64 Negative electrode active material layer 70 Separator sheet (separator)
80 nonaqueous electrolyte 100 lithium ion secondary battery

Claims (4)

preparing a negative electrode paste containing a negative electrode active material and a binder;
A step of producing a negative electrode using the negative electrode paste;
A method for producing a negative electrode of a lithium ion secondary battery comprising
The ratio (ζ _B /ζ _A ) of the zeta potential (ζ _B ) of the binder to the zeta potential (ζ _A ) of the negative electrode active material is 3.5 or more and 9.0 or less.
A method for producing a negative electrode for a lithium ion secondary battery. 2. The production method according to claim 1, wherein the negative electrode active material has a zeta potential (ζ _A ) of −3.1 mV or more and −5.5 mV or less. 3. The manufacturing method according to claim 1, wherein the negative electrode paste further contains ceramic particles exhibiting a zeta potential of -25 mV or less in the pH range of 8-9. A negative electrode for a lithium ion secondary battery comprising a negative electrode active material and a binder,
The average diameter of the binder is 0.2 μm or more and 0.5 μm or less,
A negative electrode for a lithium ion secondary battery, wherein the particle size distribution curve of the binder has a peak half width of 0.40 μm or more and 0.65 μm or less.

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